Brass and copper do not react with each other chemically under normal circumstances. These two materials can be placed in direct contact indefinitely without initiating a chemical transformation. Any perceived degradation when these materials are used together is not a direct reaction but an electrochemical process mediated by the surrounding environment.
Defining Copper and Brass
Copper is classified as a pure element, denoted by the symbol Cu. It is a reddish-brown metal existing naturally as a single substance, and its properties are intrinsic to its atomic structure. This purity makes it a consistent material with predictable characteristics, such as high electrical and thermal conductivity.
Brass, in contrast, is an alloy, which is a mixture of two or more elements where at least one is a metal. The primary components of brass are copper and zinc (Zn). Specific alloys may also include small amounts of other elements like lead, tin, or aluminum.
The proportion of copper in brass typically ranges from 60% to 90%, with the remaining percentage being zinc. An alloy is a solid solution where the constituent metals are mixed at an atomic level, not chemically bonded into a new compound. This difference in composition—a pure element versus an atomic mixture—explains their long-term stability in contact.
Stability of Metallic Bonds
The reason copper and brass do not react is rooted in the nature of their internal structure, specifically metallic bonding. In both pure copper and the copper-zinc alloy of brass, the atoms are held together by a “sea of delocalized electrons.” These valence electrons are shared freely throughout the entire crystal lattice, not tied to any single atom.
This shared electron structure means both materials are already in a state of high thermodynamic stability when placed together. There is no driving force, such as the need to form a new compound, to initiate a chemical reaction between the two metallic surfaces. The atoms on the surface of the copper and brass simply exist in stable contact.
The metallic bond is strong and requires significant external energy, such as the heat of a furnace, to break and reform. Touching the two materials together at room temperature does not provide the energy required to change their atomic structure. This inherent stability ensures that the materials are completely compatible in a dry, neutral environment.
When Environmental Factors Cause Degradation
Copper and brass do not react chemically, but they can participate in an electrochemical process called galvanic corrosion. This degradation occurs when a specific environmental condition is met: the presence of an electrolyte. An electrolyte is a conductive liquid, such as moisture, tap water, saltwater, or acidic rain.
When brass and copper are in electrical contact and submerged in an electrolyte, they form a galvanic couple, essentially creating a small battery. Because they are dissimilar metals, they possess slightly different electrical potentials, which are measured on the galvanic series. The less noble (anodic) metal will preferentially corrode to protect the more noble (cathodic) metal.
In a copper-brass system, pure copper is generally slightly more noble than brass, particularly alloys with higher zinc content. Zinc is an active metal, and its presence makes the brass the more anodic material in the couple. Consequently, the brass sacrifices itself, corroding at an accelerated rate while the copper remains protected.
Surface Area Ratio
Engineers are mindful of the ratio of the surface areas of the two metals in contact. If a small brass fixture is used with a large copper pipe system, the corrosion current is focused intensely on the small anodic brass piece. This leads to a much faster rate of material loss for the brass. Conversely, if the exposed area of the copper (cathode) is small relative to the brass (anode), the corrosion is spread out and becomes negligible.
In common plumbing applications involving potable water, the difference in nobility between copper and brass is often small. The corrosion rate is usually insignificant, provided the water is not highly aggressive or contaminated. However, in harsh environments like marine settings or industrial applications, the electrolyte significantly accelerates the galvanic process.